1. Field of the Invention
The present invention relates to a negative electrode for a lithium secondary battery, to a method for preparing the negative electrode, to a lithium secondary battery that has the negative electrode, and to a vehicle that has such a lithium secondary battery. More specifically, the present invention is directed to a negative electrode for a lithium secondary battery that has a configuration in which a negative electrode active substance layer that contains a negative electrode active substance is supported on a negative electrode collector, to a method for preparing the negative electrode, to a lithium secondary battery that has the negative electrode, and to a vehicle that has such a lithium secondary battery.
2. Description of the Related Art
The importance of secondary batteries such as lithium secondary batteries and nickel hydrogen batteries for use as a power source for vehicles or as a power source for personal computers and portable terminals has been increasing in recent years. In particular, lithium secondary batteries which are light weight and provide a high energy density is promising for use as a preferred high output power source for mounting on vehicles. In such lithium secondary batteries, charging and discharging occur through migration of lithium (Li) ions between positive and negative electrodes.
As a typical configuration of an electrode of such a lithium ion battery, there may be mentioned a structure in which an electrode active substance that is capable of reversibly occluding and releasing Li ions is formed on an electrode collector. As a negative electrode collector for use in a negative electrode, there may be mentioned, for example, a sheet-like or foil-like member that is composed mainly of copper or a copper alloy. Examples of the negative electrode active substance for use in a negative electrode include carbonaceous materials such as graphite.
However, because graphite intercalates one Li atom for every six carbon atoms, the charging and discharging capacity thereof is limited to 372 mAh/g at maximum. Various studies have thus been made on negative electrode active substances which are expected to be capable of achieving a charging and discharging capacity that is greater than that of graphite. For example, a study is made to achieve a high battery capacity by using, as a negative electrode active substance, a metal, such as tin or silicon, that forms an alloy with lithium (lithium alloy) (for example, international publication No. 00/42669). Incidentally, the use of carbon nanowall as a negative electrode material (negative electrode active substance) is disclosed in KITADA, Norio, “Application of Carbon Nanowall to Negative Electrode Material for Lithium Ion Secondary Battery”, Abstracts of Exchange Meeting of Monodukuri Gijutsu 2008, [online], Nov. 13, 2008, Kanagwa Industrial Technology Center, [searched Jul. 16, 2009], Internet <URL:http://www.kanagawa-iri.go.jp/kitri/kouhou/program/H20/poster.html>, although, this technology is not aimed at an increase of the battery capacity. As technical documents that disclose a carbon nanowall, there may be mentioned, for example, Japanese Patent Application Publication No. 2008-239369 (JP-A-2008-239369) and Japanese Patent Application Publication No. 2008-24570 (JP-A-2008-24570).
When a lithium alloy such as tin or silicon is used as a negative electrode active substance, however, various problems are caused due to a volume change of the negative electrode active substance, because such an alloy expands and shrinks more than graphite does during charging and discharging procedures. That is, in an electrode group in which a negative electrode 5 (which has a structure in which a negative electrode active substance layer 7 that contains a negative electrode active substance 6 in the form of particles is supported on a negative electrode collector 8) and a positive electrode 4 (which has a structure in which a positive electrode active substance layer 3 that contains a positive electrode active substance is supported on a positive electrode collector 2) are wound via separator 9, as shown in FIG. 12, when the volume of the negative electrode active substance 6 is reduced at the time of discharging, the thickness (volume) of the negative electrode active substance layer 7 is reduced by a load applied to the electrode group as shown in FIG. 13. As a result, the structure of the negative electrode active substance layer 7 collapses so that electrical conduction paths between materials are broken. More particularly, as a consequence of collapse of the structure of the negative electrode active substance layer 7, the contact between the negative electrode active substances 6 or between the negative electrode active substance 6 and the collector 8 is broken. This results in a reduction of the current collecting efficiency of the negative electrode 5. Further, when the thickness of the negative electrode active substance layer 7 is reduced by the shrinkage of the negative electrode active substance 6, an available space into which the negative electrode active substance 6 can re-expand at the time of charging is reduced. The negative electrode active substance 6, therefore, fails to smoothly occlude Li ions. This may cause a reduction of the battery capacity.